The present application relates to an organic electroluminescent device and a display apparatus. In particular, the present application relates to a red light-emitting, organic electroluminescent device and a display apparatus using the same.
In recent years, a display apparatus using an organic electroluminescent device (so-called “organic EL device”) is watched as a lightweight flat panel type display apparatus with high efficiency.
The organic electroluminescent device which configures such a display apparatus is provided on a transparent substrate composed of, for example, a glass and is prepared by stacking an anode composed of ITO (indium tin oxide: transparent electrode), an organic layer and a cathode in this order from the substrate side. The organic layer has a configuration in which a hole injection layer, a hole transport layer and an electron transporting light-emitting layer and further an electron transport layer or an electron injection layer are stacked in this order from the anode side. In the thus configured organic electroluminescent device, an electron injected from the cathode and a hole injected from the anode are recombined in the light-emitting layer, and light which is generated during this recombination is extracted from the substrate side via the anode.
In addition to an organic electroluminescent device having the foregoing configuration, the organic electroluminescent device also includes a so-called top emission type which is configured by stacking a cathode, an organic layer and an anode in this order from a substrate side and in which by further constituting an electrode positioned in an upper portion (an upper electrode as the cathode or anode) by a transparent material, light is extracted from the upper electrode side on an opposite side to the substrate. In particular, in a display apparatus of an active matrix type which is prepared by providing a thin film transistor (TFT) on a substrate, a so-called top emission structure in which an organic electroluminescent device of a top emission type is provided on the substrate having TFT formed thereon is advantageous in view of enhancing an aperture ratio of a light-emitting portion.
Now, in the case of taking into consideration practical implementation of an organic EL display, not only an enhancement of light extraction by widening the aperture of an organic electroluminescent device but an enhancement of luminous efficiency of the organic electroluminescent device is necessary. Then, various materials and layer configurations for the purpose of the luminous efficiency have been investigated.
For example, so far as a red light-emitting device is concerned, there has been proposed a configuration in which a naphthacene derivative (including rubrene derivatives) is used as a dopant material with respect to a new red light-emitting material in place of a pyran derivative represented by DCJTB which has hitherto been known. Also, a material having good electron transport properties such as Alq3 is used for an electron transport layer adjacent to a light-emitting layer containing such a red light-emitting material (see, for example, JP-A-2000-26334 and JP-A-2003-55652).
In the foregoing display apparatus, in order to perform full-color display, organic electroluminescent devices of respective colors which undergo emission of the three primary colors (red, green and blue) are aligned and used, or a white light-emitting organic electroluminescent device and color filters or color conversion layers of respective colors are combined and used. Of these, from the viewpoint of light extraction efficiency of light-emitting light, the configuration using organic electroluminescent devices which undergo emission of the respective colors is advantageous.
However, in the emission of the red light-emitting device using the foregoing naphthacene derivative (rubrene derivative), the current efficiency is about 6.7 cd/A, and the light-emitting color was concerned with orange emission rather than red emission.
Also, many red light-emitting layer hosts exhibit strong hole transport properties. For that reason, even in a configuration in which an electron transport layer configured by using an electron transporting material such as the foregoing Alq3 is provided adjacent to a light-emitting layer, a hole-electron recombination region easily exceeds the light-emitting layer and spreads into the electron transport layer. According to this, a lowering of the luminous efficiency in the light-emitting layer is generated. Also, in the case where emission is generated in the electron transport layer due to the hole-electron recombination, a color purity of the light-emitting light is lowered. Furthermore, in case of an electron transport material which is easily deteriorated by excitation, when the electron transport material is excited due to the hole-electron recombination in the electron transport layer, a lowering of a life characteristic is caused.
Then, it is desirable to provide a red light-emitting, organic electroluminescent device having high luminous efficiency and color purity and satisfactory life characteristic and a display apparatus using the same.